Future wireless and/or cellular systems are expected, apart from many other aspects, to offer increased coverage, support higher data rates or a combination of both. In addition, the cost aspect of building and maintaining the system is expected to become even more important in the future. As data rates and/or communication distances are increased, the problem of increased battery consumption also needs to be addressed.
One aspect is rethinking the topology used in existing systems, as there has been little change of topology over the three generations of cellular networks.
For instance, it is well known that multi-hopping offers possibilities of significantly reduced path loss between communicating (relay) entities, which may benefit the user.
Another type of topology considers so called cooperative relaying. This is a research area that investigates cooperation among multiple stations. In recent research literature, it goes under several names, such as cooperative diversity, cooperative coding, virtual antenna arrays, and so forth. A good general overview of cooperative communication schemes is given in [1]. The general benefits of cooperation between stations in wireless communication can be summarized as higher data rates, reduced outage (due to various forms of diversity), increased battery lifetime and extended coverage (e.g. for cellular systems).
More recently, the idea of cooperative relaying with smarter repeaters (or relays) has received some interest. The idea is that relays can cooperate in forwarding a signal from a transmitter to a receiver [2]. A signal sent by an originating transmitting node may first be received by a number of relays that concurrently forward the signal to a receiving node. The cooperation may for instance involve aspects of coherent combining, STC (Space-Time Coding such as Alamouti diversity), and be of regenerative (decode-and-forward) or non-regenerative (amplify-and-forward) nature. In cooperative relaying, the number of hops is normally two hops, i.e. one hop to the relay station(s), and one hop to the receiving station.
The concept of cooperative relaying may in a sense be regarded as a degenerated case of multi-hopping involving only two hops, but at the same time generalized to and allowing for parallel paths as well as signal processing to be exploited. The relays are generally allowed to perform various signal processing and coding tasks that in various ways improve the overall communication performance. The benefits of the mechanisms that are exploited in cooperative relaying may broadly be divided into diversity gain, beamforming gain and spatial multiplexing gain.
Many fundamental principles have been developed for cooperative relaying schemes [1], [3], [4], and [5], each with its own benefits and drawbacks. Cooperative relaying schemes that perform well usually rely on precise channel state information, but then require fast protocols and impose additional overhead. Alternatively, the requirements on detailed knowledge of the channel are relaxed, but then the overall communication performance is generally reduced.